Mitosis Review• Interphase• Prophase• Metaphase• Anaphase• Telophase • Cytokinesis

MITOSIS

Meiosis ”gametes”, with half the number of chromosomes, are produced.

During Meiosis diploid cells are reduced to haploid cells

Diploid (2n) Haploid (n)

If Meiosis did not occur the chromosome number in each new generation would double…. The

offspring would die.

MEIOSISInterphase

Meiosis I

Meiosis II

Metaphase IProphase I

Anaphase ITelophase I

Telophase II

Prophase IIMetaphase IIAnaphase II

2n

2n

2n4n

nnnn

Interphase/ DNA replicates

Meiosis I

Meiosis II

Prophase I - Synapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Homologous Chromosomes(because a homologous pair consists of 4 chromatids it is called a “Tetrad”)

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Meiosis I• Interphase –replication• Prophase I - homologous chromosomes pair

– Crossing over! - creates variation (diversity)

chiasmata: site of crossing over

Tetradnonsister chromatids

Meiosis I

• Metaphase I– Homologous pair line up at center (tetrad)– INDEPENDENT ASSORTMENT OCCURS:

1. Orientation of homologous pair is random2. **Variation**

produces 2n distinct gametes, where n = the number of unique chromosomes.

In humans, n = 23 and 223 = 6,000,000

Meiosis I

• Anaphase I– Homologous pairs move to opposite sides of cell

• Telophase I/Cytokinesis– Cell divides

Quick Questions

1. How many cells are at the end of Meiosis 1?

2. How many chromosomes are in each cell at the end of meiosis 1?

2n

2n

2n4n

nnnn

Interphase/ DNA replicates

Meiosis I

Meiosis II

Meiosis II• Prophase II - no DNA replication!!!• Metaphase II• Anaphase II• Telophase II/Cytokinesis

Quick Questions

1. How many cells are at the end of Meiosis 2?

2. How many chromosomes are in each cell at the end of meiosis 2?

http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

Sweet Animation!!!!!!!!!!

How many cells are at the end of meiosis? How many chromosomes are in each cell?

Agenda for Thursday April 10th 1. Finish meiosis

Meiosis Mitosis

Two cell divisions

Half the original chromosomes

Homologous Chromosomes pair up

Cytokinesis

Four daughter cells

46 Chromosomes at end

Mitosis vs. Meiosis1. Number of Cells

– Mitosis creates 2 identical cells– Meiosis creates 4 individual cells

2. Number of chromosomes– Mitosis creates cells with 46 chromosomes (2n – diploid)– Meiosis creates cells with 23 chromosomes (n – haploid)

3. Locations– Mitosis – somatic (“normal”) cells– Meiosis – gametes (sex) cells

• The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis

Mitosis The first (and distinguishing) division of meiosis

Whiteboard Questions

1. If a cell has 40 chromosomes, how many are in each cell at the end of meiosis 1?

2. Meiosis 2?3. Mitosis?4. T/F: Mitosis creates 2 different cells5. T/F: A difference between meiosis & mitosis

is that meiosis occurs in gametes and mitosis occurs in somatic cells.

Meiosis Mitosis

Chromosome Number: Reduced by half (haploid) Remains the same (diploid)

Pairing of Homologues: Yes No

Function: sexual reproductionCellular (asexual) Reproduction; general growth and repair of the body

#of Divisions: 2 1

#of Cells produced: 4 2

Creates: Sex cells (Gametes) Somatic Cells

Crossing Over: Yes No

Genetically: different identical

Importance of Meiosis

• Genetic Variation

• Random distribution of chromosomes

• Chromosomes are passed to offspring independently of each other

Another (better) Sweet Animation!!!!!!!!!!!!!!!!!!

http://wps.prenhall.com/esm_freeman_biosci_1/7/1948/498784.cw/index.html

Fertilization

• The fusion of a sperm and egg to form a zygote.• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

Asexual reproduction

• Asexual reproduction involves a single parent. • genetically identical offspring • All prokaryotes and some eukaryotes • can be very rapid

– allows them to crowd out other organisms that reproduce more slowly

Three types of asexualBinary fission - a parent cell splits into two identical daughter cellsFragmentation - a parent organism breaks into pieces and each piece develops into a new organismBudding - a parent cell forms a bubble-like bud. The bud stays attached to the parent cell while it grows and develops. When the bud is fully developed, it breaks away from the parent cell and forms a new organism.

Sexual Reproduction

• involves two parents• parents produce gametes • Gametes are haploid cells• two gametes unite is called fertilization • The fertilized cell is a zygote . • A zygote is diploid cell

Sexual vs. Asexual ReproductionAsexual

• Organisms inherit all chromosomes from single parent– Genetically identical

• Little energy• No mate needed• Good for stable env’t• Bacteria

Sexual

• Chromosomes from 2 parents

• Genetic variation– Good for changing env’t

• Most animals/organisms

Why reproduce sexually?Greater chance of beneficial mutations• Crossing over, random assortment

Check the correct box for each description

Meiosis Mitosis

1. Two cell divisions

2. Occurs in gametes

3. Half the original chromosomes

4. Homologous Chromosomes pair up

5. Creates identical cells

6. Four daughter cells

7. Same #of chromosomes at end

1. Prophase I2. Metaphase I

3. Anaphase I4. Telophase/Cytokinesis I

1. Prophase IICells from Meiosis I 2. Metaphase II

3. Anaphase II

4. Telophase II/Cytokinesis II